As part of the EU Framework Programme 7 the Advanced GaN packaging project, which ran from 1 October 2010 to 31 March 2012, targeted packaging for space applications to enable Europe to maintain a strong position in the highly competitive space industry. Research into the diamond-silver base plate material is highlighted in the published collaborative work between Bristol University, Plansee SE, United Monolithic Semiconductors and Thales Alenia Space.1 It presented a new packaging solution for GaN power electronics for efficient heat extraction needed for high-power devices, illustrating for the first time the impact of using diamond-silver composite as a base plate in packages on the self-heating of GaN devices.
Diamond-silver composites consisting of diamond particles in a matrix of silver alloy feature an excellent thermal conductivity as high as 650 W/mK at room temperature and a CTE close to that of the semiconductor materials. This is significantly larger than the thermal conductivity of conventional packaging materials such as CuW.
For the study, Micro-Raman thermography measurements were performed on AlGaN/GaN multi-finger HEMTs (power bars, with 18 fingers) grown on SiC substrates to determine their device temperature at various power levels. The devices were attached to both diamond-silver composite and CuW base plates by using standard AuSn solder, in order to assess the difference between the two materials in terms of thermal management efficiency. Figure 9 shows the temperature results from Raman measurements in the center finger of the devices at different power values, i.e., the peak device temperature rise with respect to the temperature at the backside of the base plate which was kept at 25°C.
Devices attached on diamond-silver composite base plates exhibit peak temperatures approximately half that of the peak temperatures exhibited by devices mounted on CuW base plates, especially at high-power levels, which are standard for device operation. The same trend was obtained for temperatures in the outer fingers. However, due to the crosstalk effect, temperatures are higher in the center finger.
Also, there was clear improvement from the point of view of heat extraction, with obvious benefits for device reliability and system requirements. A 3D finite element model of the device was built to compare to the experimental data and the outcomes from the simulations were consistent and in good agreement with the ones obtained experimentally. Figure 10 illustrates the temperature distribution across the whole device on diamond-silver compared to the one on the CuW base plate. Again the temperature is significantly lower in devices brazed onto the diamond-silver composite.
The final report summary for the project said: “State-of-the-art results have been obtained for L-Band HPA, putting forward the strong impact of base plate material to be used in RF power modules. Up to 65 percent PAE with up to 180 W RF power in L-Band has been obtained with no tuning on several power modules using diamond-silver base plate material. The gain in power added efficiency is about 10 points for the same design implemented into CuW standard micropackage.”
- M. Faqir, T. Batten, T. Mrotzek, S. Knippscheer, L. Chalumeau, M. Massiot, M. Buchta, J. Thorpe, H. Blanck, S. Rochette, O. Vendier and M. Kuball, “Novel Packaging Solutions for GaN Power Electronics: Silver-Diamond Composite Packages,” CS MANTECH Conference, May 2010.